However, I would base it on the fact that the B7 were slightly better built due to many learned lessons from the B6s. I'd therefore stick with 2006 and up (stay away from the 2005.5 bastard children as well).
Sprint Blue B7 S4 Avant 6MT - 421whp/350wtq
JHM Stage 1 SC - JHM SS - JHM IM - JHM IS - JHM Stg 4 clutch - JHM LWFW - 19" O.Z. Ultraleggera - Stoptech ST-60 BBK - KW v3 CO - Apikol rear diff mount - Trexturk DP - Magnaflow CB - Hotchkis RS4 RSB - Apikol snub mount - Stern Motor mounts - 034 Adj FUCA - RS4 Pedals - TT-RS MFSW - Zada-Tech Gauges - STE Gauges - Dynavin D99 Android
The platform was designed specifically for the B6. Meaning, the headlights, taillights, bumpers etc were were designed from scratch and meant to flow with the doors and roof line. Very understated and slab sided.
The B7 is an example of a refresh meant to bring the aging platform inline with the aesthetics of the other newer models. The new grill, headlights, taillights, bumpers, etc look tacked on and made to work with the B6's design. Sort of like the Golf and Jetta Citi in Canada.